Real-Time Kinematics (RTK) refers to a surveying system and method which uses two GPS receivers and a communications link between them to determine a position of one receiver relative to the other receiver. In a typical RTK survey system, a first GPS receiver is located at a known position, often a surveyor's landmark or benchmark, or an otherwise surveyed position, and the pseudorange data it collects is sent to the second GPS receiver, often referred to as a “rover,” via a radio communications link. The rover is used to determine the relative position of desired points according to the needs of the survey effort. Thus there is a radio transmitter associated with the first receiver, called a reference receiver or a base station receiver, and a radio receiver at the rover. Pseudorange data from the satellites in view from the first receiver at the base station location is combined with data taken at the second rover receiver, and is processed at the rover according to well-known RTK methods to develop a position of the rover relative to the base station position. The typical distance from base to rover for acceptable accuracy is in the range of 15-30 Km. The errors are due to atmospheric variations which in turn cause changes in the measured path length to the satellites (pseudoranges). Thus for surveying or other positioning systems which must work over larger regions, the surveyor must either place additional base stations in the regions of interest, or move his base stations from place to place. This range limitation has led to the development of more complex enhancements that have superceded the normal RTK operations described above, and in some cases eliminated the need for a base station GPS receiver altogether. This enhancement is referred to as the “Virtual Reference Station” system and method. It is also referred to as Networked RTK.
Network RTK typically uses three or more GPS reference stations to collect GPS data and extract information about the atmospheric and satellite ephemeris errors affecting signals within the network coverage region. Data from all the various reference stations is transmitted to a central processing facility, or VRS control center for Network RTK. Suitable software at the control center processes the reference station data to infer how atmospheric and/or satellite ephemeris errors vary over the region covered by the network. The control center computer processor then applies a process which interpolates the atmospheric and/or satellite ephemeris errors at any given point within the network coverage area and generates a pseudorange correction comprising the actual pseudoranges that would be received at the specified base station location, now referred to as the Virtual Reference Station, and pseudorange corrections applicable to the region surrounding that location. As in the basic RTK method, where the pseudoranges measured at the base station are delivered to the rover via a radio link, so must the corrected pseudoranges calculated at the VRS control center be delivered to the rover, or rovers doing the actual surveying. Since the VRS control center can be located anywhere, it is not likely to be within normal radio transmission range of the rovers. Typically, prior art delivery methods included using a direct cellular connection between the VRS control center and each rover.
As VRS methods have been accepted and have become widely used, more and more rovers are employed in a survey, the necessity of a dedicated direct connection for each rover has emerged as a financial burden and therefore a limitation of the VRS system. For example, a construction site may have a plurality of roving GPS units working at the same time, each with a dedicated cellular connection to the control center. Additional GPS receivers may be used with construction equipment to, for example, dig a trench for utility lines in an exact location, or grade a road in a particular location. Thus, a plurality of cellular connections may be needed within a relatively compact area for each GPS receiver being used.
Current methods for delivering VRS data to roving mobile position determination units are inadequate. Accordingly, a need exists for a VRS data distribution system and method that reduces the need for dedicated cellular telephone connections between the various rovers and a distant VRS control center.